Integrated circuits are made up of millions of active devices formed in or on a substrate, such as a silicon wafer. The active devices are chemically and physically connected into a substrate and are interconnected through the use of multilevel interconnects to form functional circuits. Typical multilevel interconnects comprise a first metal layer, an interlevel dielectric layer, and sometimes a third and subsequent metal layer. Interlevel dielectrics, such as doped and undoped silicon dioxide (SiO2) and/or low-κ dielectrics, are used to electrically isolate the different metal layers. As each layer is formed, typically the layer is planarized to enable subsequent layers to be formed on top of the newly formed layer.
Tungsten is increasing being used as a conductive material to form the interconnections in integrated circuit devices. One way to fabricate planar tungsten circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as titanium or tantalum and/or a diffusion barrier layer such as titanium nitride or tantalum nitride. The adhesion-promoting layer and/or the diffusion barrier layer are then over-coated with a tungsten layer. Chemical-mechanical polishing is employed to reduce the thickness of the tungsten over-layer, as well as the thickness of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive tungsten forming the circuit interconnects.
Typically, chemical-mechanical polishing compositions for polishing tungsten-containing substrates comprise compounds capable of etching tungsten. The compounds capable of etching tungsten, or etchants, serve to convert tungsten into a soft oxidized film that is capable of controlled removal by mechanical abrasion. Abrasion is performed using abrasives suspended in a liquid carrier to form a polishing slurry in conjunction with a polishing pad or with abrasives fixed to a polishing pad, in which movement of the polishing pad relative to the substrate (i.e., a semiconductor wafer) with the polishing slurry therebetween causes mechanical removal of the soft oxidized film. However, the etchants often are capable of converting tungsten metal or its oxide directly into soluble forms of tungsten. In the polishing step, the over-coating layer of tungsten is removed to expose the oxide layer and to achieve planarity of the substrate. After exposure of the oxide layer and before completion of the polishing process, tungsten in the trenches undesirably can be eroded by a combination of static etching and by mechanical action of the abrasives, leading to dishing and erosion. Dishing may compromise circuit integrity and leads to surface non-planarity, which may complicate deposition of metal layers on subsequent levels of the device. Inhibitors of tungsten etching have been added to chemical-mechanical polishing compositions. For example, U.S. Pat. No. 6,273,786 discloses a chemical-mechanical polishing process comprising a tungsten corrosion inhibitor selected from the group consisting of phosphates, polyphosphates, silicates, and mixtures thereof. U.S. Pat. No. 6,083,419 discloses a chemical-mechanical polishing composition comprising an inhibitor of tungsten etching that is a compound selected from the group consisting of nitrogen-containing heterocycles without nitrogen-hydrogen bonds, sulfides, and oxazolidines.
However, such inhibitors are not always effective at preventing erosion of tungsten within trenches. Additionally, use of high levels of such inhibitors of tungsten etching can reduce the polishing rates of substrates comprising tungsten layers to unacceptably low levels. Erosion is a function not only of tungsten etching, but also of the abrasion process. Thus, there remains a need in the art for compositions and methods for chemical-mechanical planarization of tungsten-containing substrates that will provide for reduced erosion of tungsten and yet maintain useful rates of tungsten removal. The invention provides such a chemical-mechanical polishing composition and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.